Process for the hot isostatic compression of silicon nitride bodies reinforced with carbide fibres and carbide whiskers

ABSTRACT

Carbide fiber- and carbide whisker-strengthened silicon nitride bodies are hot-isostatic pressed without encapsulation of the pressureless pre-sintered formed body at temperature between 1000° and 3000° C. in a nitrogen-protective gas atmosphere. At a low nitrogen partial pressure but high total gas pressure, a maximum consolidation can be achieved already at relatively low temperatures without destruction of the carbide fibers or carbide whiskers.

FIELD OF THE INVENTION

The invention concerns a process for hot-isostatic pressing (HIP) ofcarbide fiber- and carbide whisker-strengthened silicon nitride bodies.

BACKGROUND OF THE INVENTION

Because of their outstandingly good properties with regard to heat- andoxidation-stability, silicon nitride ceramics have great importance asmaterials in engine construction, e.g. for the use in heat engines, aswell as as forming and cutting tools in the case of metal working. Theelectrical and mechanical properties can thereby be furthersubstantially improved by the introduction of carbide fibers or carbidewhiskers (U.S. Pat. No. 4,507,224). Therefore, the production of siliconnitride ceramics strengthened with carbide fibers or carbide whiskers isof great importance.

The product of whisker-strengthened Si₃ N₄ ceramics to give densematerials with improved mechanical properties, as are aimed forespecially for engine construction, previously took place only by hotpressing or hot-isostatic pressing of encapsulated powder bodies.However, hot pressing is limited to bodies with simple, uniformgeometry.

P. D. Shalek et al., Am. Ceram. Soc. Bull. 65 (1986), 351-265, describethe production of Si₃ N₄ strengthened with SiC whiskers without hotpressing at 1600° to 1850° C., whereby dense bodies were achieved withup to 40 vol. % whisker proportion. However, this hot-isostatic pressingrequires an encapsulation and the removal thereof after theconsolidation.

Without encapsulation of the powder body, in the case of the process ofhot-isostatic pressing, a decomposition of the carbide fibers andwhiskers takes place according to the reaction metalcarbide+nitrogen→metal nitride+carbon (metal=Si, Hf, Nb, Zr, Ta, Ti, V)which leads to a loss of the property-improving action of the fiber andwhisker incorporation.

On the other hand, the high sinter temperature require, however,increased nitrogen pressures in order to prevent a decomposition of theSi₃ N₄ matrix into silicon and nitrogen. Therefore, it is necessary, inthe case of pressureless sintering, to use temperatures below 1900° C.in order to avoid a decomposition of the Si₃ N₄ material in the case ofthe sintering according to the reaction.

    Si.sub.3 N.sub.4 (solid→3 Si (liquid)+2 N.sub.2 (gas) (1)

In the case of gas pressure sintering (pressures in the range of up tosome 10 MPa) and even more in the case of hot-isostatic pressing (up tosome 100 MPa) in a nitrogen atmosphere, silicon nitride remains stableup to very high temperatures (e.g. up to above 2700° C. at 100 MPa).

By gas pressure sintering at 1700° to 2000° C. with an N₂ pressure of 10bar, there could admittedly be produced also bodies with variablegeometry with up to 20 wt.-% SiC whiskers but, for a completeconsolidation, a proportion of up to 35 mole % sinter adjuvants is thennecessary (Tamari et al., YogYo-Kyokai-Shi 94 (1986), 1177-1179).However, this high proportion of sinter adjuvants has the disadvantagethat it leads to the formation of a correspondingly high glass portionin the sintered material which manifests itself in a drastic impairmentof the mechanical high temperature properties.

However, in the case of hot-isostatic pressing of carbide fiber- orcarbide whisker-strengthened silicon nitride, there must also be takeninto account the decomposition of the carbide fibers or whiskersaccording to the above already-given equation "metalcarbide+nitrogen→metal nitride+carbon". Therefore, for the totalstability of the ceramic body, the decomposition reaction of Si₃ N₄ andthe decomposition reaction of the carbide fibers are to be taken intoaccount, where these two reactions stand in a mutual relationship anddisadvantageously influence the stability of the other component inquestion (Si₃ N₄ or carbide fiber).

OBJECT OF THE INVENTION

Therefore, it was the task of the present invention to make available aprocess for hot-isostatic pressing of carbide fiber- and carbidewhisker-strengthened silicon nitride bodies with which theabove-mentioned disadvantages can be avoided and with which, in simpleand economic manner, carbide fiber- and whisker-strengthened siliconcarbide bodies can be produced which, on the basis of their mechanical,chemical and electrical properties and on the basis of their stability,fully satisfy the high requirements.

DESCRIPTION OF THE INVENTION

It has now been found that one can avoid the disadvantages of theprevious processes and especially the decomposition of the components(Si₃ N₄ and carbide fibers), when one works in a nitrogen-protective gasatmosphere at a high total gas pressure and a particular nitrogenpartial pressure.

Therefore, the subject of the invention is a process for hot-isostaticpressing of silicon nitride formed bodies strengthened with carbidefibers or carbide whiskers from the group SiC, HfC, NbC, TaC, TiC, VC orZrC in a nitrogen-containing atmosphere, wherein the formed body,presintered pressureless in a nitrogen-containing atmosphere, is,without encapsulation, hot-isostatic pressed at temperatures between1000° and 3000° C. in a nitrogen-protective gas atmosphere under anitrogen partial pressure which corresponds to the equation ##EQU1## inwhich P_(N2) signifies the N₂ partial pressure, T_(S) is the temperaturein °K, and e is the natural logarithm base, where the partial pressurelower limit is given by the values a=872 213, b=405.6 and C--16.6 andthe partial pressure upper limit, depending upon the carbide used, bythe following values:

    ______________________________________                                               a           b       c                                                  ______________________________________                                        SiC:     505444        295.2   -16.6                                          HfC:     159842        83.2    -4.2                                           NbC:      95814        73.4    -4.2                                           TaC:     106274        81.2    -4.2                                           TiC:     143782        76.1    -4.2                                           VC:      114641        67.9    -4.2                                           ZrC:     160552        80.9    -4.2                                           ______________________________________                                    

WO-A-86/05480 discloses a process for pressureless sintering of ceramicbodies which are reinforced with carbide whiskers. The ceramic bodiesmanufactured according to this process can subsequently be subjected tohot-isostatic pressing in a nitrogen or argon atmosphere. Forpre-sintering, temperatures up to 1750° C. and for hot-isostaticpressing temperatures up to 1800° C. are indicated. However, examplesare only given therein for ceramic bodies based on aluminum oxide forwhich a hot-isostatic pressing process is carried out at a temperatureof 1575° C. The special problems which occur during hot-isostaticpressing (HIP) of an Si₃ N₄ form body containing carbide fibers are notmentioned. The process described therein does not provide a solution tothese problems.

When sintering, one cannot omit nitrogen as the gas since, in case of areduction of the nitrogen partial pressure, the upper limit of thesinter temperature drops drastically on the basis of equation (1).

The nitrogen partial pressure, which is to be adjusted for thehot-isostatic pressing of carbide fiber- or whisker-strengthened Si₃ N₄composite material without destroying matrix or fiber/whisker must,therefor, vary between an upper and a lower limit for the carbide inquestion. It has now been found that the particular limit can be givenby the equation ##EQU2## where T_(S) is the sinter or HIP temperature in°K (K).

The values for the parameters a, b and c of equation (2) for the lowerlimit of the nitrogen partial pressure at which the Si₃ N₄ is stillstable at a given sinter temperature and which, therefore, must not begone below is given by equation (10) below. Therefore, the lower limitof the nitrogen partial pressure is the same for all materials of thedescribed type and has the parameters

    a=872 213; b=405.6; C=-16.6

The upper limit of the nitrogen partial pressure which can be used isgiven by the reactions (equations b 3 to 9)

    ______________________________________                                        2 HfC + N.sub.2 → 2 HfN + 2 C                                                                (3)                                                     2 NbC + N.sub.2 → 2 NbN + 2 C                                                                (4)                                                     3 SiC = 2 N.sub.2 → 3 Si.sub.3 N.sub.4 + 3 C                                                 (5)                                                     2 TaC + N.sub.2 → 2 TaN + 2 C                                                                (6)                                                     2 TiC + N.sub.2 → 2 TiN + 2 C                                                                (7)                                                     2 VC + N.sub.2 → 2 VN + 2 C                                                                  (8)                                                     2 ZrC + N.sub.2 → 2 ZrN + 2 C                                                                (9)                                                     ______________________________________                                    

and must, therefore, be individually determined for each carbide.

Table 1 gives the parameters a, b and c from which the upper limit ofthe nitrogen partial pressure (P_(N2)) for the carbide used according toequation (2) can be calculated. These values are known from theliterature (cf. Chase et al., (1075), JANAF thermochemical tables;Storms, E. K. (1967), The refractory carbides, Refractory materials 3,Academic Press, N.Y., U.S.A.; Toth, L. E. (1971) Transition metalcarbides and nitrides, Refractory materials 7, Academic Press, N.Y.U.S.A.).

                  TABLE 1                                                         ______________________________________                                        Parameters a, b and c for the calculation of the upper limit                  of the usable nitrogen partial pressure                                              a           b       c                                                  ______________________________________                                        SiC:     505444        295.2   -16.6                                          HfC:     159842        83.2    -4.2                                           NbC:      95814        73.4    -4.2                                           TaC:     106274        81.2    -4.2                                           TiC:     143782        76.1    -4.2                                           VC:      114641        67.9    -4.2                                           ZrC:     160552        80.9    -4.2                                           ______________________________________                                    

According to the process of the invention, one works in the temperaturerange of 1000° to 3000° C. The upper temperature limit for theproduction of the composite ceramic is determined by the stability ofthe silicon nitride according to the reaction equation (1). It can beexpressed by the equation

    T (°C.)=(872 213/(405.6-16.6 ln (P.sub.N2))-273     (10)

where the nitrogen partial pressure P_(N2) is given in bar.

The lower temperature limit is determined by the stability in thepresence of nitrogen of the carbide in question which is to be used asfiber or whisker. This means that the reactions according to theequations (3) to (9) must be avoided

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a to 1g graphically illustrate the possible nitrogen partialpressures (P_(N2)) and temperature (T) conditions in each case for thecarbides to be used at which one can work according to the invention.The cross-hatched area illustrates the possible region.

They show:

FIG. 1a the P_(N2) -T region for HIP of NbC fiber/whisker-strengthenedSi₃ N₄ ;

FIG. 1b the P_(N2) -T region for HIP of TaC fiber/whisker-strengthenedSi₃ N₄ ;

FIG. 1c the P_(N2) -T region for HIP of SiC fiber/whisker-strengthenedSi₃ N₄ ;

FIG. 1d the P_(N2) -T region for HIP of HfC fiber/whisker-strengthenedSi₃ N₄ ;

FIG. 1e the P_(N2) -T region for HIP of TiC fiber/whisker-strengthenedSi₃ N₄ ;

FIG. 1f the P_(N2) -T region for HIP of ZrC fiber/whisker-strengthenedSi₃ N₄ ;

FIG. 1g the P_(N2) -T region for HIP of VC fiber/whisker-strengthenedSi₃ N₄ ;

In FIGS. 1a-1g, temperature is plotted against N₂ partial pressure(bar);

In FIG. 2 density (g./cm³ is plotted against proportion of whiskers(vol. %); and

in FIG. 3 K_(IC) (MPa√m) is plotted against proportion of whickers (vol.%).

As a protective gas for the nitrogen-protective gas atmosphere any gasor gas mixture which is inert under the operating conditions can beused. The protective gas is preferably a noble gas, such as Ar, He, Ne,Xe or a mixture thereof.

The composition of the nitrogen-protective gas atmosphere is given asfollows:

    % nitrogen=100×P.sub.N2 /P.sub.Hip ;

and

    % nobel gas=100-100×P.sub.N2 /P.sub.Hip,

where P_(Hip) (bar) is the total pressure desired or necessary fortechnical reasons in the case of hot-isostatic pressing, the size ofwhich lies in the usual pressure range employed for the process for thehot-isostatic pressing. In the process according to the invention, oneusually works with a total gas pressure P_(Hip) of at least 1000 bar.

The proportion of carbide fibers and/or carbide whiskers can be to up to50 wt.-%, referred to the starting mixture. The lower limit of thecarbide portion is, as a rule, about 0.1 wt. %, preferably 0.5 wt. % andespecially 1 wt. %. The upper limit of the carbide portion preferably is30 wt. % and especially 15 wt. %.

The starting mixture (powder body) for the process according to theinvention can also contain additive materials and melt phases usual forsuch processes for the production of silicon nitride formed bodies, suchas conventional sinter adjuvants (sinter additives). Preferred sinteradjuvants are Y₂ O₃, Al₂ O₃ and AlN, and especially powder mixturesthereof, or also Y₃ Al₅ O₁₂. The proportion of sinter additivespreferably increases with increasing carbide content. In particular, theproportion of sinter additives is 5 to 15 wt. % (referred to thestarting mixture), where the lower limit of 5 wt. % is preferablyemployed for the lower limit of the carbide proportion and the upperlimit of 15 wt. % preferably for the upper limit of the carbideproportion. When Y₂ O₃, Al₂ O₃ and AlN are used as a powder mixture, theratio of the sinter additive powder preferably lies in the limits: Y₂ O₃80 to 40 wt. %; Al₂ O₃ 10 to 30 wt. % and AlN 10 to 30 wt. %.

The process according to the invention is combined for hot-isostaticpressing with a preceding pressureless pre-sintering in a nitrogenatmosphere, preferably up to the achievement of a closed porosity(greater than 5%). The pre-sintering can take place in a known and usualway and with use of conventional additive materials, such as sinteradjuvant agents (sinter additives). Preferably, the above-mentionedsinter additives and especially powder mixtures of Y₂ O₃, Al₂ O₃ and AlNor Y₃ Al₅ O₁₂ are used. The preferably used proportion of sinteradditives also corresponds to the above-mentioned preferred proportionranges.

Sinter additives strongly reduce the mechanical properties of theceramic, especially at high temperatures. In the case of sintering underincreased gas pressure or in the case of processes which intend apost-treatment by hot-isostatic pressing, less of the additive materialsare necessary in order to achieve a maximum density of a ceramic body.With the process according to the invention, it is now possible to keepthe amount of additive materials, such as sinter adjuvant agents, low. Afurther advantage of the process according to the invention also consistin the fact that in the case of lower nitrogen partial pressure but hightotal gas pressure, a maximum consolidation can already be achieved atrelatively low temperatures.

The following Example explains the invention in more detail withoutlimiting it thereto.

EXAMPLE

A powder mixture consisting of 85 to 70 wt. % Si₃ N₄, up to 15 wt/%β-SiC whiskers and 9.8 wt. % Y₂ O₃, 1.7 wt. % Al₂ O₃ and 3.5 wt. % AlNis homogeneously mixed by attrition in an organic liquid. Thewhisker-containing powder suspension is subsequently dried in a rotaryevaporator and cold-isostatic pressed into sample bodies. The greendensity amounts to 2-2.1 g./cm³ (60-63% of theoretical density).

The composite ceramic is pre-sintered for 30 minutes at 1850° C. underan N₂ pressure of 1 bar (0.1 MPa) up to closed porosity. The densityafter pre-sintering reaches 3.3 to 3.15 g/cm³ for whisker contents of upto 15 wt. % (99 to 95% of theoretical density).

Subsequently, the pre-sintered sample body is capsulelesshot-isostatically post-consolidated (HIP process) at a temperature of1900° C. for 10 minutes in a gas mixture of 1 vol. % N₂ and 99 vol. %Ar. The isostatic total pressure amounts to 1000 bar (100 MPa), wherethe N₂ partial pressure reaches 10 bar (a MPa). The density alsoincreases in the case of samples with up to 15 wt. % SiC whisker contentto above 3.3 g./cm³ (99% of the theoretical density) without adestruction of the SiC whiskers taking place in the sample body. FIG. 2shows graphically the absolute density and relative density (fractionaldensity in %) in relation to the whisker proportion.

If, on the other hand, one post-consolidates with an N₂ partial pressureof 1000 bar (100 MPa) (which corresponds to the above total pressure), astrong carbon formation with simultaneous destruction of the SiCwhiskers on the sample surface is observed.

After the hot-isostatic post-consolidation, a distinct increase in thebreak resistance is recognized. FIG. 3 shows the K_(IC) values inrelation to the whisker proportion. Maximum values of the K_(IC) of over8.5 MPa m^(1/2) are achieved with β-SiC whisker content of 10 wt. %.

In FIGS. 2 and 3, the curve (S) means the pre-sintering, the curve (HIP)the hot-isostatic post-consolidation.

Upon replacement of the SiC whiskers by HfC, NbC, TaC, TiC, VC or ZrCwhiskers, working according to the above-given method and with the sameamount proportions produced similar results are obtained.

We claim:
 1. The method of hot isostatic pressing of a shaped siliconnitride body reinforced with carbide fibers or carbide whiskers selectedfrom the group consisting of SiC, HfC, NbC, TaC, TiC, VC or ZrC in anitrogen-containing atmosphere, which comprises subjecting said shapedbody to pressureless presintering in a nitrogen-containing atmosphere,hot isostatically pressing said presintered body without encapsulationat a temperature between 1000° and 3000° C. in a protective gasatmosphere under a nitrogen partial pressure which corresponds to theequation ##EQU3## wherein P_(N).sbsb.2 is the N₂ partial pressure, T_(S)is the temperature in °K, and e is the natural logarithm base, where thepartial pressure lower limit is given by the values a=872 212, b=405.6and c=-16.6 and the partial pressure upper limit, depending upon thecarbide which is used, is given by the following values

    ______________________________________                                               a           b       c                                                  ______________________________________                                        SiC:     505444        295.2   -16.6                                          HfC:     159842        83.2    -4.2                                           NbC:     95814         73.4    -4.2                                           TaC:     106274        81.2    -4.2                                           TiC:     143782        76.1    -4.2                                           VC:      114641        67.9    -4.2                                           ZrC:     160552        80.9    -4.2.                                          ______________________________________                                    


2. The method of claim 1, wherein the protective gas is a noble gas or anoble gas mixture.
 3. The method of claim 1, wherein the total gaspressure of nitrogen and protective gas is at least 1000 bar (100 MPa).4. The method of claim 1, wherein the amount of carbide fibers orcarbide whiskers in said silicon nitride body is up to 50 wt.-%, basedon the starting mixture.
 5. The method of claim 1, wherein thepressureless presintering is carried out in a nitrogen atmosphere up toclosed porosity.
 6. The method of claim 1, which is carried out in thepresence of a known sintering additive selected from the groupconsisting of Y₂ O₃, BeO, Li₂ O, MgO, CaO, BaO, SrO, Sc₂ O₃, Al₂ O₃,ZrO₂ and mixtures thereof.
 7. The method of claim 6, wherein the amountof sintering additive is 5 to 15 wt.-%, based on the starting mixture.